A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. In general, a so-called scanner comprises a positioning device for positioning the patterning device and a positioning device for positioning the substrate. Such positioning devices usually comprise one or more linear motors for displacing the patterning device or substrate and an object table for holding the patterning device or substrate. In order to increase the number of substrates exposed per unit of time, more efficient and powerful positioning devices are applied enabling a higher acceleration of the object tables holding the patterning device or substrate. As a consequence of the increased acceleration, inertial forces acting on the object that is held on the object table (e.g. the patterning device or the substrate) are also increasing. A similar increase in inertial force can also be noticed in case a larger substrate or patterning device is desired. A consequence of the increased inertial force, it becomes more and more difficult to hold the patterning device or substrate on the object table. Conventionally, holding the substrate or patterning device is done by friction. A Sufficient level of friction can be applied by the use of vacuum chambers. In case the requirements with respect to acceleration of the object table are more demanding, the application of only a vacuum clamping may not be sufficient. As a result, the patterning device or substrate may slip on the object table resulting in a possible overlay error. Some improvements have been suggested in literature to increase the holding force that holds the patterning device or substrate. Such improvements may encompass the use of additional clamping using electrostatic forces or magnetic forces as indicated in U.S. Pat. No. 6,469,773. Further improvements indicated in U.S. Pat. No. 6,469,773 describe the use of a mechanism comprising a lever and a weight to exert a force on the patterning device during acceleration. Due to the introduction of such a mechanism, an inferior dynamic behavior of the object table may occur. This may result in an increase in settle-time (i.e. the time desired to have the position of the object table within the desired error bandwidth). The mechanism as described may introduce an additional weight to be accelerated by the object table. Thus, it is desirable to provide a higher force to obtain the desired acceleration of the patterning device or substrate. As a consequence, the accuracy of the positioning of the object may become affected.
US 2003/0197841 also describes some possible arrangements to increase the holding force. The described arrangements ensure an increased holding force by increasing the surface that is subject to the vacuum or reduced pressure or by introducing an addition force perpendicular to the surface of the patterning device. With such an arrangement, it may be desirable to have additional space above the patterning device or substrate. Often, this space is not available. Furthermore, increasing the holding force by clamping the object on upper or lower surface or by exerting additional forces along a side or sides of the object may result in an unwanted deformation of the object causing inaccuracies in the projected image.